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The moment when this at-times ponderous fifth season of "Mad Men" finally started to takeoff, when the seemingly endless and aimless build-up began to make sense, came during the eighth episode, in the back of a cab.

That’s where Don Draper gushed in uncharacteristic awe over his young, beautiful new wife Megan, the secretary he impulsively wed and molded in his own advertising-whiz image, after her quick thinking saves the big Heinz account. "You know,” he tells her, “you're good at all of it."

That’s the moment she realizes she must cleave herself from her husband’s office life, a move that turns Draper’s briefly stable married-to-his-work world upside down – leading to last week's stunning episode and setting the stage for Sunday’s potentially explosive season finale.

The AMC drama’s theme of one man’s identity crisis appeared, in the season’s early going, to extend to the show, which seemed in slow-motion flux. But “Mad Men” creator Matthew Weiner’s deliberate wind has uncoiled into an ultimately satisfying season in which several major characters, to different degrees, lost themselves trying to be Don Draper.

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Megan, unsatisfied as a clone of her husband, proved bold enough to rebel and pursue her acting dream. Copywriter Peggy Olson, another secretary Draper tried to turn into a female version of himself, emerged as more Don-like inside the office and out (her anonymous sexual encounter in a movie theater) – but finally left the firm so she could be herself (or at least try out her Draper Jr. act elsewhere).

Power hungry junior partner Pete Campbell desperately wishes to be like the old Draper, enjoying the hedonistic life while juggling a family in the suburbs – but a brief, destructive affair leaves him more miserable. Even Draper’s adolescent daughter, Sally, wants to be part of his world, though she becomes disillusioned after a sordid night out with the advertising crowd in the “dirty” city. Office manager Joan Harris, taking Draper’s old win-at-all-costs salesman credo literally, sells herself to join him as a partner at Sterling Draper Cooper Pryce.

And ineffectual Lane Pryce extracts from himself the ultimate price after being caught impersonating Draper by forging his signature on a check. "Do you know how the rest of us live?" an exasperated Pryce asks Draper when he’s found out, his British once-stiff upper lip quivering.

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Meanwhile, the season gave us an almost unrecognizable, semi-domesticated Don Draper, albeit one still struggling with his urges, simmering rage and past secret life. At his unwanted surprise 40th birthday party – marking the day the real Don Draper was born – he bristles at Megan’s sexy “Zou Bisou Bisou” song-and-dance number, the viral pop-cultural moment of the season and the first pen stroke in her declaration of independence. He’s no longer an active ladykiller, but finds himself hallucinating about strangling a woman from his past who, in his imagination, threatens to destroy his new, steady life. He's showing signs of losing his creative edge, as he sabotages an underling's superior campaign for his own run-of-the-mill idea on how to hawk Sno-Cones.

Draper is out of touch and he senses it. Sure, the firm is running a successful “Hard Day’s Night”-themed commercial. But times are changing quickly – it’s 1966, after all. Megan gives Don a copy of the Beatles’ new “Revolver” album and points him to the proto-psychedelic track “Tomorrow Never Knows.” He doesn’t dig the song, originally titled “The Void” – which is exactly what Don finds himself starring into after Megan leaves the firm and elevator doors open to reveal an empty shaft. It marked the season’s most surreal scene – save, perhaps, for Roger Sterling’s loopy acid trip.

There’s a different, very real kind of void created by departure of Lane, whose office suicide may have been presaged in the controversial falling-man ads that heralded the season. But as Sunday’s finale approaches, it’s Don Draper who stands at the edge of a new abyss as he grapples with guilt over pushing Lane too far. It may have taken the man who used to be “good at all of it” a while to get this point. But it’s been worth the wait to see where he and the show will leap next as 1967 and the Summer of Love near. Because, as the Beatles told us, tomorrow never knows.